1. Field of the Invention
The present invention relates generally to a position detecting system and position detecting method using a plurality of position pointing devices.
2. Description of the Prior Art
As one of the position detecting modes on a digitizer there is what is known as an electromagnetic-signal transmitting and receiving mode. For example, this comprises a position detecting surface including a number of loop coils arranged parallel to each other, and a pointing device, such as a pen and cursor, wherein the loop coils are used as sensors, i.e., antennas, so that transmission and reception of electromagnetic waves are carried out between the loop coils and pointing device to sense a coordinate of the pointing device and other information on the basis of the resulting detection signal. One important feature of this mode is providing a cordless pointing device which therefore does not require any cord. As disclosed in Japanese Patent Publication No. 2-53805 and Japanese Patent Laid-Open No. 3-147012, the present applicant has proposed a variety of digitizers based on this electromagnetic signal transmitting and receiving mode. One of the major problems in these applications relates to how to avoid noises and to accurately and quickly obtain information on the basis of the reception signal.
In such an electromagnetic signal transmitting and receiving mode, a transmission and reception operation, in which, for example, an electromagnetic wave is transmitted from one selected antenna which in turn again receives an electromagnetic wave re-transmitted from a resonance circuit or coil within the pointing device, is repeatedly practiced with the antennae being changed in turn, so that a calculation containing interpolation is made on the basis of the signal from the antenna attendant with the strongest reception signal and the signals from the antennae close thereto, thereby determining the coordinate of the pointing device.
In addition, cordless position detecting systems using a plurality of such pointing devices have been well-known, for example, Japanese Patent Laid-Open Nos. 63-70326, 63-108424, 63-108425, 63-108426. In such systems, resonance frequencies of resonance circuits of two pointing devices are set to be different from each other so that the position detection is accomplished by using the resonance frequency of the resonance circuit of one pointing device subjected to coordinate detection to avoid interference from the resonance circuit of the other pointing device.
Moreover, a cordless position detecting system has also been known which uses a pointing device containing batteries. Such a pointing device containing batteries is based on the operational principle that an electromagnetic wave is one-sidedly transmitted from the pointing device. For using a plurality of pointing devices on such an operational principle, the simplest way is to have the frequencies of the electromagnetic waves to be transmitted from the respective pointing devices adapted to be different from each other so as to implement reception at every frequency.
On the other hand, a new method of use has been proposed wherein a cordless pointing device is not limited to only an input device, but is used as a means to store, display and output information (for example, as disclosed in Japanese Patent Laid-Open No. 5-181582). In this case, it is required to accomplish the position instruction operation and information communication at a cordless state.
For information exchange with a specific one of a plurality of pointing devices to be simultaneously used, a means should be needed which is capable of selectively establishing communication with the specific pointing device. Particularly, a transmission means which is capable of transmitting information to only the specific pointing device is important. On the other hand, for reception of information from the specific pointing device, there is no particular problem, because control may be made so as to perform the transmission by only the specific pointing device.
As a way of implementing the selective communication with a specific plurality of pointing devices, a method of assigning different inherent electromagnetic-wave frequencies to the respective pointing devices is known. However, in this instance, difficulty is experienced in simultaneously using a plurality of pointing devices to which the same frequency is assigned. For this reason, combinations of pointing devices to be simultaneously used are subjected to limitations, thus causing the users to be thrown into confusion.
This problem also applies to the case of employing a resonance circuit of a pointing device in common as a means to make communication for the purpose of coordinate detection of the pointing device. That is, in a case where the resonance frequencies of resonance circuits of pointing devices to be used simultaneously are the same, it is impossible to similarly perform both the position detection and communication. For this reason, there is the need for different frequencies being assigned in advance to resonance circuits of a plurality of pointing devices, and therefore, as well as the above, combinations of pointing devices to be simultaneously used are limited, thus resulting in the users being brought to confusion.